Studies are being conducted on the process of phagocytosis of outer segment membranes by the pigment epithelium, with emphasis on control mechanisms of this process. Two separate experimental methodologies have been developed to study this problem. In the first, bovine retinas have been radioactively labeled and the outer segments separated and fed to chick pigment epithelium cell cultures. The time course of ingestion of the outer segments can be monitored with a liquid scintillation counter and has been verified by electron microscopy and histochemistry. By placing appropriate sugars or hormones with the outer segments, the effect of these agents on phagocytosis can be quantified. In the second approach, normal and dystrophic rat eyecups are incubated in vitro in a solution containing radioactive sugars and/or melatonin. Both autoradiography and phagosome counts are made to see if the sugars are instrumental in preparing the outer segments for shedding, and to see if the melatonin plays a role in controlling this process. BIBLIOGRAPHIC REFERENCES: Goldman, A.I., Ham, W.T., Jr., and Mueller, H.A.: Ocular damage thresholds and mechanisms for ultrashort pulses of both visible and infrared laser radiation in the rhesus monkey. Exp. Eye Res. 24: 45-56, 1977. Geeraets, W.J., Ham, W.T., Jr., Geeraets, R., and Goldman, A.I.: Photochemical, thermal, and non-linear effects of retinal irradiation, In L'Esperance Francis, (ed.), Current Diagnosis and Management of Chorioretinal Diseases. St. Louis, Mosby, 1977, pp. 9-24.